HUMAN BITES and FIGHT BITES
The Clenched Fist Injury | Eikenella corrodens
Injury Patterns
Critical Must-Knows
- A 'Fight Bite' is a Septic Arthritis until proven otherwise.
- The tooth penetrates the MCPJ capsule in FLEXION.
- When the finger extends, the tract is sealed by the extensor hood gliding proximally.
- This traps bacteria (Eikenella, Strep, Staph) in the joint.
- Eikenella corrodens is resistant to First Gen Cephalosporins (Keflex) and Clindamycin.
- Treatment is emergent formal arthrotomy and washout.
Examiner's Pearls
- "Never trust a 'small cut' over the MCPJ in a young male.
- "Cephalexin alone effectively TREATS the bacteria it covers but SELECTS OUT Eikenella.
- "Augmentin is the drug of choice.
The Clinical Trap
The Lie
"I cut it on a fence/can/glass" Patients often lie about the mechanism due to shame or legal reasons. A laceration over the 3rd/4th/5th MCPJ is a fight bite until proven otherwise. X-ray may reveal a tooth fragment or 'Boxer's Fracture' (Neck of 5th MC).
The Consequence
Joint Destruction If treated as a simple laceration (sutured), the septic arthritis proceeds unchecked. Cartilage destruction (chondrolysis) occurs within 24-48 hours. Osteomyelitis and amputation are real risks.
| Feature | Animal Bite | Human Bite |
|---|---|---|
| Organism | Pasteurella / Capnocytophaga | Eikenella / Strep Viridans / Staph |
| Mechanism | Puncture / Crush | Inoculation into Joint (CFI) |
| Antibiotic | Augmentin | Augmentin (But Ceph/Clinda fail) |
| Urgency | Urgent | Emergent (if Joint involved) |
HACEKEikenella Corrodens
Memory Hook:Eikenella is part of the HACEK group (Endocarditis risk).
FISTAction Plan
Memory Hook:Don't miss the FIST injury.
AUGAntibiotics
Memory Hook:AUGmentin AUGments the cure.
Overview
A "Human Bite" encompasses two distinct injuries: the Occlusal Bite (direct clamp) and the Clenched Fist Injury (CFI). The CFI is the most dangerous, involving the inoculation of oral flora directly into the MCP joint or extensor apparatus by striking a tooth.
Human saliva contains up to 10 billion (10^9) bacteria per mL, with a diverse mix of aerobes and anaerobes. The unique "gliding" mechanism of the extensor tendon over the MCP joint acts as a one-way valve, sealing the inoculum deep within the joint once the fist is relaxed. This creates a closed-loop abscess within the joint capsule that destroys cartilage (chondrolysis) within 48 hours.
Pathophysiology and Mechanisms
The "Sliding Target"
- Impact: Fist is clenched (MCPJs flexed 90 degrees). Extensor tendon and hood move distally over the metacarpal head.
- Penetration: Tooth penetrates skin, tendon, capsule, and enters joint.
- Relaxation: Fist opens (MCPJs extend). Extensor tendon glides proximally.
- Sealing: The skin wound is now proximal to the capsule wound. They are no longer aligned.
This misalignment creates an anaerobic environment within the joint and prevents drainage.
Microbiology
Eikenella corrodens
- Characteristics: Gram-negative facultative anaerobe.
- Prevalence: Found in 25-30% of human bite infections.
- Synergy: Acts synergistically with Strep viridans to worsen infection.
- Resistance Profile:
- Resistant: Clindamycin, Erythromycin, First Gen Cephalosporins (Cephalexin), Metronidazole (Variable).
- Sensitive: Penicillin, Ampicillin, Augmentin, Ciprofloxacin, Ceftriaxone.
Treating a human bite with Keflex (Cephalexin) is a classic error.
Classification Systems
Injury Types
- Occlusal Bite: Direct clamping. Often finger tip or soft tissue areas. Can cause amputation.
- Clenched Fist Injury (CFI): Indirect inoculation. High risk to joint/tendon.
- Self-Inflicted: Nail biting (Paronychia), psychogenic.
Classification guides urgency. CFI = Urgent Arthrotomy. Occlusal = Debridement.
History
Screening
- The Story: Often vague ("cut on metal").
- Hand Position: Was the hand a fist?
- Time: Delayed presentation is common (pain increases overnight).
- Tetanus/Vaccination: Status.
Assume any laceration over the MCPJ is a fight bite.
Examination
Examination
- Location: Usually 3rd (Middle) or 4th (Ring) MCPJ (prominent heads).
- Wound: Often small (3-5mm). May be sealed.
- ROM: Pain on passive motion (Septic Arthritis).
- Tendons: Assess extensor mechanism stability (sagittal band injury).
- Neurovascular: Digital nerves.
Look for "gas" or crepitus (rare but ominous - necrotizing fasciitis).
Investigations
Radiology
- X-Ray: Mandatory.
- Findings:
- Fracture: 5th Metacarpal Neck (Boxer's Fracture) - suggests punch mechanism.
- Foreign Body: Tooth fragment (radiopaque).
- Air: Gas in the joint (Pathognomonic for penetration).
- Osteomyelitis: Late finding (periosteal reaction, osteopenia, erosions).
MRI is rarely indicated in the acute setting but useful for chronic osteomyelitis. ULTRASOUND can also be used to guide aspiration of joint effusions if the diagnosis is unclear.
Clinical Presentation
Management Strategy
Strategic Approach
- Admit: IV Antibiotics.
- Explore: Surgical exploration is mandatory for CFI.
- Wash: Arthrotomy and irrigation.
- Leave Open: Do not suture.
- Rehab: Early motion once infection controlled.
Early mobilization prevents stiffness and tendon adhesion.
Surgical Considerations
Formal Arthrotomy
- Incision: Extend the laceration (usually transverse or Z-plasty). Avoid longitudinal crossing of joint creases.
- Exposure: Expose the extensor tendon. Retract it to inspect the capsule. A longitudinal split of the tendon (splitting the sagittal fibers) may be needed to visualize the joint surface directly.
- Capsulotomy: If a tear is found, open it. If no tear is found but clinical suspicion is high, open it. The joint MUST be visualized.
- Irrigation: Copious saline (e.g. 3-6 Liters). Use a 18G catheter on a syringe for jet lavage within the joint.
- Staging: Inspect the metacarpal head for "divots" (tooth impact) or cartilage loss.
- Closure: Leave skin open. Loose approximation only if very clean.
A strict "No Closure" policy is safest for the junior surgeon.
Antibiotic Protocol
Gold Standard: Augmentin (Amoxicillin + Clavulanate). Why? Covers Staph, Strep, Anaerobes, AND Eikenella.
Penicillin Allergy:
- Ciprofloxacin (Covers Eikenella) OR
- TMP/SMX (Bactrim)
- PLUS Clindamycin or Metronidazole (for Anaerobes/Staph).
Remember: Clindamycin ALONE misses Eikenella. Cephalexin ALONE misses Eikenella.
Complications
Infection-Related Complications
| Complication | Risk Factors | Prevention | Management |
|---|---|---|---|
| Septic Arthritis | MCP joint penetration, delay | Early I&D, IV antibiotics | Joint washout, may need multiple |
| Osteomyelitis | Metacarpal head involvement, delay | Early debridement | Prolonged IV antibiotics, debridement |
| Deep Space Infection | Palmar involvement, immunocompromised | Aggressive exploration | I&D of web space, thenar space |
| Necrotizing Fasciitis | Diabetes, delayed presentation | High index of suspicion | Emergent radical debridement |
Structural Complications
- Tendon Rupture: May occur from initial injury or secondary to infection. Extensor tendons most vulnerable at MCP level.
- Extensor Subluxation: Sagittal band disruption allows tendon to sublux between metacarpal heads. May need surgical repair.
- Joint Stiffness: Common after septic arthritis. Early mobilization crucial once infection controlled.
- Contractures: Collateral ligament shortening if immobilized in extension. Splint in intrinsic plus position.
Worst-Case Scenarios
In severely neglected cases with immunocompromise or significant delay:
- Amputation: May be required for uncontrollable infection or extensive tissue loss
- Hand function loss: Permanent grip weakness and finger stiffness
- Systemic sepsis: Life-threatening if infection spreads
Rehabilitation
- Splinting: Volar splint in intrinsic plus (MCPs flexed 70, IPs extended) to prevent collateral ligament shortening.
- Elevation: High elevation in a Bradford sling to reduce edema.
- Dressings: Saline soaked gauze to encourage wicking of purulence.
- Motion: Start Active Range of Motion (AROM) as soon as cellulitis resolves and the wound is clean.
- Tendon: Isolate Extensor Digitorum Communis (EDC) gliding exercises.
- Wound: Secondary intention healing (granulation).
- Scar Management: Desensitization once healed.
- Strengthening: Grip strengthening once soft tissue coverage is complete.
Prognosis
Outcomes by Presentation Time
| Timing | Infection Rate | Joint Outcome | Overall Prognosis |
|---|---|---|---|
| Early (less than 24h) | 10% | Good if no joint penetration | Excellent with appropriate treatment |
| Late (24-48h) | 30-40% | Moderate risk of stiffness | Fair with aggressive management |
| Delayed (greater than 48h) | 50%+ | High risk of permanent damage | Guarded, multiple surgeries likely |
Injury Pattern and Prognosis
- Simple bite wound: Good prognosis with early antibiotics
- Clinched fist injury (CFI): Worse prognosis due to joint penetration and contamination
- Joint involvement: Even with treatment, 20-30% develop some permanent stiffness
Factors Predicting Poor Outcome
Negative prognostic factors include:
- Delay in presentation: Greater than 24 hours significantly worsens outcomes
- MCP joint penetration: Risk of septic arthritis and cartilage destruction
- Eikenella corrodens: Often resistant to empiric therapy, may be missed
- Patient comorbidities: Diabetes, immunosuppression, alcoholism
- Incomplete debridement: Retained contamination leads to persistent infection
Long-Term Functional Outcomes
Most patients treated appropriately within 24 hours return to full hand function. Those presenting late or with joint involvement may experience:
- Reduced grip strength (20-40%)
- Limited MCP range of motion
- Cold intolerance
- Persistent pain
Evidence Base
Microbiology of Human Bites
- Multicenter prospective study
- Eikenella corrodens present in 30% of human bites
- Resistance to Clindamycin/Cephalexin confirmed
Early vs Late Treatment
- Comparison of patients presenting less than 24h vs greater than 24h
- Significant increase in complications, osteomyelitis, and amputation in delayed group
- Emphasizes urgency
Surgical Debridement
- Review of 345 cases
- Formal arthrotomy reduced hospital stay and permanent disability compared to antibiotics alone or simple I&D
Viral Transmission
- Low risk of HIV transmission via saliva unless blood present
- Higher risk for Hepatitis B
- Recommended HBV prophylaxis/vaccine
Primary Closure?
- Studied facial human bites
- Primary closure acceptable for facial bites less than 24h post-debridement
- NOT applicable to hand/CFI
Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: The 'Cut on a Tooth'
"A 24-year-old male presents with a 4mm laceration over his right long finger MCPJ. He says he cut it on a tooth during a fight 2 days ago. It is red, swollen, and he cannot extend the finger due to pain. He has been taking Keflex from his GP."
Scenario 2: The Boxer's Fracture
"X-ray shows a Boxer's fracture (5th MC Neck) with volar angulation. On clinical exam, there is a small scab over the 5th MCPJ. Patient denies a bite."
Scenario 3: Eikenella
"Why is Eikenella corrodens significant in human bites? What is its unique resistance profile?"
MCQ Practice Points
Microbiology
Q: Which bacteria is characteristically resistant to Clindamycin and Cephalexin in human bites? A: Eikenella corrodens.
Anatomy
Q: In a Clenched Fist Injury, the bacterial inoculum is trapped because: A: The extensor tendon glides proximally upon finger extension, sealing the tract.
Antibiotics
Q: What is the first-line oral antibiotic for a human bite? A: Amoxicillin + Clavulanate (Augmentin).
Pathology
Q: What is a 'Honeymoon Period' in flexor tenosynovitis? A: The period (12-24h) where the bacteria are proliferating but signs are subtle, before rapid escalation.
Treatment
Q: What is the mandatory surgical approach for a confirmed clenched fist injury? A: Formal arthrotomy with copious irrigation (3-6L saline) and the wound left open.
Complications
Q: What complication should you suspect if a fight bite patient presents with crepitus? A: Necrotizing fasciitis - requires emergent radical debridement.
Australian Context
Australian Epidemiology
Human bite injuries, particularly fight bites, are common presentations to Australian emergency departments, especially on weekends and after major sporting events. The majority occur in young males aged 18-35 years, often associated with alcohol intoxication.
Under-reporting is common due to social stigma and medico-legal concerns surrounding assault. Early presentation improves outcomes significantly, with delayed presentations (greater than 24 hours) having markedly higher complication rates.
Antibiotic Guidelines
Australian practice follows the Therapeutic Guidelines (eTG):
- First-line: Amoxicillin + Clavulanate (Augmentin) - PBS listed
- Penicillin allergy: Ciprofloxacin plus Metronidazole
- IV option: Amoxicillin/Clavulanate or Ampicillin/Sulbactam
- Duration: 5-10 days depending on severity and response
Blood-Borne Virus Considerations
Management includes assessment for blood-borne virus transmission:
- HIV/Hepatitis B/C testing: Requires informed consent from both parties if possible
- Post-Exposure Prophylaxis (PEP): Available through emergency departments if significant risk identified
- Follow-up: Baseline and repeat serology at 3 and 6 months
Documentation
Thorough documentation is essential as these injuries often have medicolegal implications:
- Time and mechanism of injury
- Clinical findings and photographs
- Treatment provided and patient compliance
- Discussion of prognosis with patient
Public Hospital vs Private Practice
In Australia, the majority of fight bite injuries are managed in public hospital emergency departments:
- Initial assessment: Emergency department triage and assessment
- Operative management: Usually performed in public hospital operating theatres
- Follow-up: Hand therapy and wound review, often in public outpatient clinics
- After-hours presentations: Common, requiring on-call orthopaedic or plastic surgery cover
Prevention and Public Health
Prevention strategies focus on alcohol-related violence reduction through responsible service of alcohol programs and public awareness campaigns regarding the severity of punch injuries.
Early presentation and appropriate treatment remain the most important factors in achieving good outcomes for patients with fight bite injuries. Patient education about the serious nature of these injuries is essential.
High-Yield Exam Summary
Diagnosis
- •Small wound over MCPJ = Fight Bite
- •Boxer's Fracture + Wound = Open/Infected
- •Pain on passive ROM = Septic Arthritis
- •X-ray: Look for Air and Tooth
Microbiology
- •Polymicrobial
- •Eikenella corrodens (Gram Neg Anaerobe)
- •Strep viridans
- •Staph aureus
Management
- •Admit + IV Augmentin
- •Formal Arthrotomy + Washout
- •Leaves Wounds Open
- •Splint in Intrinsic Plus